For starters, Quest is by far the biggest selling VR/XR headset. So he is already seeing some success here.
And as we’ve seen before, Facebook isn’t going to dominate forever. It makes complete sense throwing large sums of money at future technology while you still have large sums of money to burn.
Chasing new technology when you’re already behind and your revenue is decline is a guaranteed way to fail.
Moonshots like this you waste a tonne of money when that money is comparatively cheap anyway. Plus it keeps people talking about your business, which is never a bad thing.